Frequency shift light modulator and laser doppler measuring device

ABSTRACT

A frequency shift light modulator includes a resonator and a diffraction grating including a plurality of grooves arranged in parallel in a displacement direction of the resonator, and the diffraction grating is provided on the resonator. By providing the diffraction grating on the resonator, it is easy to realize miniaturization and increase in accuracy of the frequency shift light modulator. Further, it is easy to realize application to a high frequency region in a MHz band, that is, high frequency modulation. It is possible to efficiently obtain an effect based on a combination of the resonator and the diffraction grating.

The present application is based on, and claims priority from JPApplication Serial Number 2019-064313, filed Mar. 28, 2019, thedisclosure of which is hereby incorporated by reference herein in itsentirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a frequency shift light modulator anda laser Doppler measuring device for grasping a motion state of a movingobject.

2. Related Art

JP-A-2007-285898 is an example of the related art regarding this type offrequency shift light modulator and laser Doppler measuring device.JP-A-2007-285898 discloses that a shift amount of frequency is obtainedusing a heterodyne interference.

Specifically, in paragraph [0028] of the description, it is describedthat “For a resonator element, it is desirable to use a piezo elementthat has a property of being deformed when, for example, a voltage, amagnetization, or the like is applied, and whose resonance frequency isvariable by changing the voltage. Further, the resonance frequency isrequired to be a triangular wave or a sawtooth wave whose waveform riseslinearly. A light Doppler shift is used which is caused by incidence oflaser light when a sawtooth wave applied voltage rises or when atriangular wave applied voltage rises and falls.”

However, in general, a resonator element such as a quartz crystalresonator or a Si resonator uses a simple resonance drive having a veryhigh Q value indicating sharpness of oscillation, and the driving methoddescribed therein may not be accurate and realistic. Further, there isno description or suggestion about application to resonance in a highfrequency in a MHz band, that is, high frequency modulation.

SUMMARY

The present disclosure is directed to a frequency shift light modulatorthat includes a plate-shaped resonator that repeats a mode, by applyinga potential, in which a crystal is distorted and resonates in adirection along a plane; and a diffraction grating that is provided on asurface of the resonator and that includes a plurality of linear groovesarranged periodically.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of a laser Doppler measuringdevice according to an embodiment of the present disclosure.

FIG. 2 is a schematic perspective view of a frequency shift lightmodulator according to the embodiment.

FIG. 3 is a diagram illustrating generation of a plurality of beams ofdiffracted light from incident light in the light modulator of theembodiment.

FIG. 4 illustrates light paths of the incident light and diffractedlight in the light modulator of the embodiment.

FIG. 5 is a schematic configuration diagram illustrating that a lightsource unit, a frequency shift light modulator, and a light receivingelement are mounted on a base according to the embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

First, the present disclosure will be schematically described.

A frequency shift light modulator according to a first aspect of thepresent disclosure for solving the above problem includes a resonatorand a diffraction grating including a plurality of grooves arranged inparallel in a displacement direction of the resonator, and thediffraction grating is provided on the resonator.

According to this aspect, the diffraction grating is provided on theresonator, so that it is easy to realize miniaturization and increase inaccuracy of the frequency shift light modulator. Further, it is easy torealize application to a high frequency region in a MHz band, that is,high frequency modulation.

Further, in a case in which the frequency can be modulated, diffractedlight can have a larger modulation frequency when the resonatorresonates in a grating vector direction.

According to this aspect, since in the plurality of grooves, anextending direction of the groove intersects a direction of theresonance, it is possible to efficiently obtain an effect based on acombination of the resonator and the diffraction grating, and thus it ispossible to easily realize high frequency modulation of the frequencyshift light modulator.

According to a second aspect of the present disclosure, in the frequencyshift light modulator according to the first aspect, the diffractiongrating is a blazed diffraction grating.

According to this aspect, since the diffraction grating is the blazeddiffraction grating, it is possible to increase light use efficiency,and thus it is possible to easily realize the miniaturization andincrease in SN of the frequency shift light modulator.

According to a third aspect of the present disclosure, in the frequencyshift light modulator according to the first aspect or the secondaspect, the resonator is a quartz crystal AT resonator that performsthickness-shear resonance in the high frequency region in the MHz band.

According to this aspect, since the resonator is the quartz crystal ATresonator that performs the thickness-shear resonance in the highfrequency region in the MHz band, it is possible to easily realize theminiaturization, the increase in accuracy, and further the highfrequency modulation of the frequency shift light modulator.

A laser Doppler measuring device according to a fourth aspect of thepresent disclosure includes: a light source unit that outputs laserlight of a predetermined wavelength; a polarization beam splitter; aplurality of λ/4 plates; an analyzer; a light receiving element thatconverts light into an electrical signal; the frequency shift lightmodulator according to the first aspect; and a set unit of ato-be-measured object. The polarization beam splitter, the λ/4 plate,and the set unit are disposed on an optical axis of the laser lightoutput from the light source unit. The λ/4 plate and the frequency shiftlight modulator are disposed on an optical axis of laser light reflectedby the polarization beam splitter. The analyzer is disposed between thepolarization beam splitter and the light receiving element. Dopplershift light reflected from the to-be-measured object and frequency shiftlight reflected by the frequency shift light modulator is guided to thelight receiving element passing through the λ/4 plates, the polarizationbeam splitter and the analyzer.

According to this aspect, since the frequency shift light modulatordescribed in the first aspect is provided in the laser Doppler measuringdevice, it is possible to easily realize miniaturization, increase inaccuracy, and high frequency modulation of the laser Doppler measuringdevice.

According to a fifth aspect of the present disclosure, in the laserDoppler measuring device according to the fourth aspect, the frequencyshift light is diffracted light of ±1st order.

According to this aspect, the frequency shift light modulator isdisposed such that the diffracted light of ±1st order is used as thefrequency shift light, so that it is possible to realize theminiaturization and high stability of the measuring device.

According to a sixth aspect of the present disclosure, in the laserDoppler measuring device according to the fourth aspect, the frequencyshift light is any beam of diffracted light of ±2nd order or more.

According to this aspect, since the frequency shift light modulator isdisposed such that any beam of the diffracted light of ±2nd order ormore is used as the frequency shift light, it is possible to realize theminiaturization and high frequency modulation of the measuring device.

According to a seventh aspect of the present disclosure, in the laserDoppler measuring device according to the fifth aspect or the sixthaspect, the frequency shift light modulator is disposed such that anentering direction of the laser light to the frequency shift lightmodulator and a traveling direction of the frequency shift lightreflected from the frequency shift light modulator form an angle of 180degrees.

Here, the “angle of 180 degrees” in “the frequency shift light modulatoris disposed such that an entering direction of the laser light to thefrequency shift light modulator and a traveling direction of thefrequency shift light reflected from the frequency shift light modulatorform an angle of 180″ may be formed when reflection is performed using amirror or when a plane formed by the diffraction grating is disposed totilt without using a mirror.

According to this aspect, when the “angle of 180 degrees” is formed byusing a mirror, modulated light is subjected to Doppler shift twice, andit is possible to realize higher frequency modulation. Further, when the“angle of 180 degrees” is formed without using a mirror, the mirror isnot necessary, so that it is possible to realize furtherminiaturization.

Subsequently, the frequency shift light modulator and the laser Dopplermeasuring device according to an embodiment of the present disclosurewill be described with reference to FIGS. 1 to 5 . In the drawings, thesame components are denoted by the same reference numerals, and theindividual descriptions of the respective drawings are omitted.

Embodiment

Laser Doppler Measuring Device

A configuration of a laser Doppler measuring device 1 according to anembodiment of the present disclosure will be described with reference toFIG. 1 .

The laser Doppler measuring device 1 according to the present embodimentincludes a light source unit 2 that outputs laser light of apredetermined wavelength, a polarization beam splitter 4, two λ/4 plates6 and 8, an analyzer 9, a light receiving element 10 that converts lightinto an electrical signal, a frequency shift light modulator 12, and aset unit 16 of an to-be-measured object 14. The polarization beamsplitter 4, the λ/4 plates 6, and the set unit 16 are disposed in thisorder on an optical axis 18 of the light emitted from the light sourceunit 2. On the other hand, the λ/4 plate 8 and the frequency shift lightmodulator 12 are disposed in this order on an optical axis 20 of lightreflected by the polarization beam splitter 4. The analyzer 9 isdisposed between the polarization beam splitter 4 and the lightreceiving element 10.

Further, Doppler shift light 22 reflected from the to-be-measured object14 and frequency shift light 24 reflected by the frequency shift lightmodulator 12 are guided to the light receiving element 10 passingthrough the λ/4 plates 6 and 8, the polarization beam splitter 4, andthe analyzer 9.

Further, the polarization beam splitter may be used as anon-polarization beam splitter. In this case, the λ/4 plate is notnecessary, and the number of components is reduced, so that it ispossible to realize further miniaturization. Further, when stability ofinterference is degraded, it is preferable to design the frequency shiftlight modulator 12 in view of the degradation.

Light Source Unit

Since coherency is necessary for the light source unit 2, a laser lightsource in the MHz band with a narrow line width is used. Specifically,examples of the light source unit 2 include a gas laser represented by aHeNe laser; and a DFB-LD and a VCSEL which are semiconductor lasers thatare easy to miniaturize.

Polarization Beam Splitter

In the present embodiment, the polarization beam splitter 4 isconfigured with P-polarization transmission and S-polarizationreflection. The laser light output from the light source unit 2 isincident on a center of the polarization beam splitter 4 on the opticalaxis 18 with a ratio of P-polarization to S-polarization being 50%:50%.In the P-polarization, light passes through the polarization beamsplitter 4 and is directed toward the λ/4 plate 6 on the optical axis18. On the other hand, in the S-polarization, light is reflected by thepolarization beam splitter 4 and is directed toward the λ/4 plate 8 onthe optical axis 20.

λ/4 Plate

The P-polarized light passes through the λ/4 plate 6 and is circularlypolarized, and the moving to-be-measured object 14 is irradiated withthe circularly polarized light. The laser light by which the movingto-be-measured object 14 is irradiated is reflected as the Doppler shiftlight 22 Doppler shifted only by fd (Hz), passes through the λ/4 plate 6again and is S-polarized.

On the other hand, the S-polarized light that is reflected by thepolarization beam splitter 4 and that is directed toward the λ/4 plate 8on the optical axis 20 passes through the λ/4 plate 8, is circularlypolarized, and is incident on the frequency shift light modulator 12.The laser light incident on the light modulator 12 is reflected as thefrequency shift light 24 that is subject to frequency shift fm, passesthrough the λ/4 plate 8 again, and is P-polarized.

Analyzer

Since orthogonal polarized light is independent of each other, theinterference does not appear by only simply superimposing the polarizedlight. Here, a light wave synthesized in this way passes through theanalyzer 9 tilted at 45 degrees with respect to the both polarizedlight, and then is detected by the light receiving element 10. Thus, thepolarized light is brought into a state in which components are commonto each other, which can cause an interference phenomenon.

Light Receiving Element

The Doppler shift light 22 Doppler shifted by the moving to-be-measuredobject 14 and the frequency shift light 24 frequency-shifted by thefrequency shift light modulator 12 is guided to the light receivingelement 10 through the polarization beam splitter 4 and the analyzer 9.In the light receiving element 10, the Doppler shift light 22 and thefrequency shift light 24 is interfered (optical heterodyneinterference), and light having a frequency of fm-fd is generated. Basedon the light having the frequency of fm-fd, it is possible to obtain amotion of the to-be-measured object 14, that is, speed and resonancethereof. A photodiode or the like can be used as the light receivingelement 10.

Frequency Shift Light Modulator

A configuration of the frequency shift light modulator 12 according tothe embodiment of the present disclosure will be described withreference to FIG. 2 .

The frequency shift light modulator 12 includes a plate-shaped resonator30 that repeats a mode, by applying a potential, in which a crystal isdistorted and resonates in a direction along a plane of the resonator;and a diffraction grating 34 that is provided on a surface of theresonator 30 and that includes a plurality of linear grooves 32 arrangedperiodically. In FIG. 2 , the reference numeral 31 denotes a substrateto which the resonator is attached, a pad 33 is provided on thesubstrate 31, and further, a pad 35 is provided on a back surface of thesubstrate 31.

In the present embodiment, as for a dimension and shape of the substrate31, the substrate 31 is square with one side of 1.6 mm and a thicknessof 0.35 mm. The resonator 30 is square with one side of 1 mm and athickness of 0.07 mm, and oscillates at 24 MHz. Although an example ofthe resonator having a basic oscillation frequency of 24 MHz is shownhere, the basic oscillation frequency can be adjusted from 1 MHz to 300MHz by changing the thickness of the resonator. The diffraction grating34 has a pitch of 1 μm and a blazed angle of 25 degrees, and is providedon an entire surface of the resonator 30. The diffraction grating 34 maybe provided only on a part of the surface of the resonator 30 but notthe entire surface.

In the present embodiment, the resonator 30 is a quartz crystal ATresonator that performs thickness-shear resonance in a high frequencyregion in a MHz band. Further, the diffraction grating 34 is a blazeddiffraction grating. The blazed diffraction grating refers to adiffraction grating having a stepped cross-sectional shape.

As shown in FIG. 2 , the linear grooves 32 of the diffraction grating 34are provided such that a direction of the straight line is orthogonal toa direction 36 of the resonance of the resonator 30.

Method for Forming Diffraction Grating

An example of a method for forming the diffraction grating 34 includes amethod in which a mold is first formed by a method using mechanicalengraving (looping engine), and then the diffraction grating 34 isformed on an electrode of a chip of the quartz crystal AT resonator bynano-imprinting. Here, the reason why the diffraction grating 34 isformed on the electrode is that high-quality thickness-shear resonancecan be generated on the electrode in principle when the resonator 30 isthe AT resonator. The present disclosure is not limited to theelectrode, and the diffraction grating 34 can also be formed on anon-electrode material surface. Further, it is also possible for anelectrostatic drive type Si resonator, and a SAW device or the like togenerate high quality in-plane resonance even on the non-electrodematerial surface, so that the formation location can be selected asappropriate. Further, the diffraction grating 34 can also be formed by amethod of exposing or etching, electron light lithography, and focusedion beam processing (FIB), or the like.

Further, a metal film or a mirror film made of a dielectric multilayerfilm may be provided on a resist diffraction grating formed on the chipof the quartz crystal AT resonator, which is because reflectance of thediffraction grating 34 provided with the metal film or the mirror filmincreases.

The chip or wafer on which the resist diffraction grating is providedmay be processed by dry etching to remove the resist, and then themirror film may be provided. Therefore, effect of moisture absorption orthe like due to the resist is eliminated, so that the stability of thediffraction grating 34 can be increased. Further, by making the mirrorfilm a metal such as Au or Al, the resonator can also function as anoscillation electrode. Alternatively, a structure such as anodizedalumina (porous alumina) may be used.

Modulation Frequency Fm when Thickness-Shear Resonator Element is Used

Modulated light is subject to Doppler shift and the frequency thereof ismodulated. As shown in FIG. 3 , according to the principle of the laserDoppler, a plurality of beams of diffracted light Kns are generated fromincident light Ki incident on the diffraction grating 34 of thefrequency shift light modulator 12. Here, n is the order of thediffracted light and n=0, ±1, ±2 . . . .

In FIG. 3 , the diffraction grating 34 is not the blazed diffractiongrating in FIG. 2 , but a diffraction grating with repeatedirregularities. In actual manufacturing, the order n of the diffractedlight can be appropriately selected and used.

Although FIG. 3 shows a case in which the incident light Ki is incidentfrom a direction perpendicular to the plane of the resonator 30, thepresent disclosure is not limited to a vertical incident angle, and theincident light Ki may be obliquely incident, that is, the incident anglemay be set as appropriate. When the incident light Ki is obliquelyincident, a direction of the diffracted light changes correspondingly.

In a design of the diffraction grating, light of higher order satisfying|n|≥2 may not appear. Therefore, it is desirable to set |n|=1 in orderto obtain a stable modulation signal. That is, in the laser Dopplermeasuring device 1 of FIG. 1 , the frequency shift light modulator 12 ispreferably disposed such that diffracted light of ±1st order is used asthe frequency shift light 24.

With this arrangement, it is possible to realize the miniaturization andhigh stability of the laser Doppler measuring device 1.

When light of a higher order satisfying |n|≥2 appears in the diffractiongrating 34, the frequency shift light modulator 12 may be disposed suchthat any beam of the diffracted light of ±2nd order or more is used asthe frequency shift light 24 in the laser Doppler measuring device 1 ofFIG. 1 .

Therefore, since the diffracted light of high order can be used, it ispossible to realize high frequency modulation and the miniaturization ofthe laser Doppler measuring device 1.

In the present embodiment, the frequency shift light modulator 12 isdisposed such that an entering direction of the laser light to thefrequency shift light modulator 12 and a traveling direction of thefrequency shift light 24 reflected from the frequency shift lightmodulator 12 form an angle of 180 degrees.

A diagram at an upper side of FIG. 4 is a case in which a mirror 37 isused to form the angle of 180 degrees.

A diagram at a center of FIG. 4 corresponds to a case in which theresonator 30 is provided to tilt so as to form the angle of 180 degrees.

A diagram at a lower side of FIG. 4 is a case in which the diffractiongrating 34 is a blazed diffraction grating, and a blazed angle θ_(B) isused to form the angle of 180 degrees. The angle of 180 degrees isformed by a combination of the blazed angle θ_(B) and an incident angleβ of the incident light Ki.

Therefore, when the “angle of 180 degrees” is formed by using a mirror37, the modulated light is subjected to Doppler shift twice, and it ispossible to realize higher frequency modulation.

Further, when the “angle of 180 degrees” is formed by tilting theresonator 30 without using the mirror, since the mirror is notnecessary, it is possible to realize the further miniaturization.

When the diffraction grating 34 is a blazed diffraction grating, sincethe angle of 180 degree is formed by the combination of the blazed angleθ_(B) and the incident angle β of the incident light Ki, it is possibleto further realize the miniaturization and the high frequencymodulation.

Mounting Structure of Laser Doppler Measuring Device

FIG. 5 is a schematic configuration diagram illustrating that the lightsource unit 2, the frequency shift light modulator 12, and the lightreceiving element 10 are mounted on a base 39. In FIG. 5 , the lightsource unit 2 is mounted at a center of the base 39, the frequency shiftlight modulator 12 is mounted at a lower position of the base 39, andthe light receiving element 10 is mounted at an upper position of thebase 39, but it is needless to say that the present disclosure is notlimited to the arrangement.

Since the light source unit 2, the frequency shift light modulator 12,and the light receiving element 10 are mounted on the base 39, prisms 40and 42 are provided on the optical axis 20. Further, a convex lens 44 isdisposed between the light source unit 2 and the polarization beamsplitter 4, and the light output from the light source unit 2 iseffectively used.

In the mounting structure shown in FIG. 5 , the light receiving element10 is a photodiode having a square size of 0.1 mm, the light source unit2 is a VCSEL light source having a square size of 10 μm, and thefrequency shift light modulator 12 has a square size of 1 mm. Asdescribed above, it is possible to realize the miniaturization of themodule up to a several mm square size.

Description of Effects of Embodiments

According to the present embodiment, the diffraction grating 34 isprovided on the resonator 30, so that it is possible to realize theminiaturization and the increase in accuracy of the frequency shiftlight modulator 12. Further, it is possible to realize application tothe high frequency region in the MHz band, that is, the high frequencymodulation.

Further, the linear grooves 32 of the diffraction grating 34 have aconfiguration in which the direction of the straight line intersects thedirection 36 of the resonance of the resonator 30, so that it ispossible to efficiently obtain an effect based on the combination of theresonator 30 and the diffraction grating 34, and thus it is possible toeasily realize the high frequency modulation of the frequency shiftlight modulator 12.

Further, in the present embodiment, the diffraction grating 34 is ablazed diffraction grating, so that it is possible to increase the lightuse efficiency, and thus it is possible to easily realize theminiaturization and the increase in SN of the frequency shift lightmodulator 12.

Further, in the present embodiment, the resonator is the quartz crystalAT resonator that performs thickness-shear resonance in the highfrequency region in the MHz band. Therefore, it is possible to easilyrealize the miniaturization, the increase in accuracy, and further thehigh frequency modulation of the frequency shift light modulator 12.

Furthermore, the laser Doppler measuring device of the presentembodiment includes the frequency shift light modulator 12 according tothe present embodiment, so that it is possible to easily realize theminiaturization, the increase in accuracy, and the high frequencymodulation of the laser Doppler measuring device 1.

Other Embodiments

Although the frequency shift light modulator and the laser Dopplermeasuring device according to the present disclosure are basicallyconfigured as described above, it is possible to change or omit partialconfiguration without departing from the gist of the present disclosure.

In the above embodiment, for the diffraction grating 34, a case of usingthe quartz crystal AT resonator is described, but any element may beused as appropriate as long as the element can perform in-planeresonance such as the thickness-shear resonance. For example, inaddition to the thickness-shear resonator element, examples of theresonator include an electrostatic drive type Si-MEMS resonator or aresonator using a piezoelectric element such as a piezo. Further, asurface acoustic wave resonator (SAW device) can also be used.

What is claimed is:
 1. A frequency shift light modulator, comprising: aresonator; and a diffraction grating including a plurality of groovesarranged in parallel in a displacement direction of the resonator,wherein the diffraction grating is provided on the resonator; and theresonator is a quartz crystal AT resonator that performs thickness-shearresonance in a high frequency region from 1 MHz to 300 MHz.
 2. Thefrequency shift light modulator according to claim 1, wherein thediffraction grating is a blazed diffraction grating.
 3. A laser Dopplermeasuring device, comprising: a light source unit that outputs laserlight of a predetermined wavelength; a polarization beam splitter; aplurality of λ/4 plates; an analyzer; a light receiving element thatconverts light into an electrical signal; the frequency shift lightmodulator including; a resonator; and a diffraction grating including aplurality of grooves arranged in parallel in displacement direction ofthe resonator, wherein the diffraction grating is provided on theresonator; and a set unit of a measurement object, wherein thepolarization beam splitter, the λ/4 plate, and the set unit are disposedon an optical axis of the light output from the light source unit, theλ/4 plate and the frequency shift light modulator are disposed on anoptical axis of light reflected by the polarization beam splitter, theanalyzer is disposed between the polarization beam splitter and thelight receiving element, and Doppler shift light reflected from themeasurement object and frequency shift light reflected by the frequencyshift light modulator is guided to the light receiving element passingthrough the λ/4 plates, the polarization beam splitter, and theanalyzer.
 4. The laser Doppler measuring device according to claim 3,wherein the frequency shift light is diffracted light of ±1st order. 5.The laser Doppler measuring device according to claim 3, wherein thefrequency shift light is any beam of diffracted light of ±2nd order ormore.
 6. The laser Doppler measuring device according to claim 4,wherein the frequency shift light modulator is disposed such that anentering direction of the laser light to the frequency shift lightmodulator and a traveling direction of the frequency shift lightreflected from the frequency shift light modulator form an angle of 180degrees.